This invention relates to a method and plant for separating air.
The most important method commercially for separating air is by rectification. In such a method there are typically performed steps of compressing and purifying the air, fractionating the compressed, purified, air in a higher pressure rectification column, condensing nitrogen vapour separated in the higher pressure rectification column, employing a first stream of resulting condensate as reflux in the higher pressure rectification column, and a second stream of the resulting condensate as reflux in a lower pressure rectification column, withdrawing an oxygen-enriched liquid air stream from the higher pressure rectification column, introducing an oxygen-enriched vaporous air stream into the lower pressure rectification column, and separating the oxygen-enriched vaporous air stream therein into oxygen-rich and nitrogen-rich fractions. The condensation of nitrogen is effected by indirect heat exchange with boiling oxygen-rich liquid fraction in the bottom of the lower pressure rectification column.
The purification of the air is performed so as to remove impurities of relatively low volatility, particularly water vapour and carbon dioxide. If desired, hydrocarbons may also be removed.
At least a part of the oxygen-enriched liquid air which is withdrawn from the higher pressure rectification column is typically partially or completely vaporised so as to form the vaporous oxygen-enriched air stream which is introduced into the lower pressure rectification column.
A local maximum concentration of argon is created at an intermediate level of the lower pressure rectification column beneath the level at which the vaporous oxygen-enriched air stream is introduced. If it is desired to produce an argon product, a stream of argon-enriched oxygen vapour is taken from a vicinity of the lower pressure rectification column below the oxygen-enriched vaporous air inlet where argon concentration is typically in the range of 5 to 15% by volume, and is introduced into a bottom region of the side rectification column in which an argon product is separated therefrom. The side column has a condenser at its head from which a reflux flow for the side column can be taken. The condenser is cooled by a part or all of the oxygen-enriched liquid air withdrawn from the higher pressure rectification column, the oxygen-enriched liquid air thereby being vaporised. Such a process is illustrated in EP-A-377 117.
The rectification columns are sometimes required to separate a second liquid feed air stream in addition to the first vaporous feed air stream. Such a second liquid air stream is used when an oxygen product is withdrawn from a lower pressure rectification column in liquid state, is pressurised, and is vaporised by heat exchange with incoming air so as to form an elevated pressure oxygen product in gaseous state. A liquid air feed is also typically employed in the event that one or both the oxygen and nitrogen products of the lower pressure rectification column are taken at least in part in liquid state. Employing a liquid air feed stream tends to reduce the amount of liquid nitrogen reflux available to the rectification, particularly, for example, if a liquid nitrogen product is taken. If an argon product is produced there is typically a need for enhanced reflux in the lower pressure rectification column in order to achieve a high argon recovery. The relative amount of liquid nitrogen reflux may also be reduced by introducing vaporous feed air into the lower pressure rectification column (in which example nitrogen cannot be separated from this air in the higher pressure rectification column and is therefore not available for condensation) or by withdrawing a gaseous nitrogen product from the higher pressure rectification column, not only when liquid products are produced but also when all the oxygen and nitrogen products are withdrawn in gaseous state from the rectification columns. There may therefore be a difficulty in obtaining a high argon recovery in, for example, any of the circumstances outlined above, particularly if a liquid nitrogen or liquid oxygen product is produced. Accordingly, it may be necessary, for example, to sacrifice either production or purity of liquid products (including liquid product streams that are vaporised downstream of their exit from the rectification columns) and any gaseous nitrogen product that is taken from the higher pressure rectification column or recovery of argon.